1. Field of the Invention
This invention relates to a metallic gasket for use in sealing a clearance between the opposed surfaces of a cylinder head and a cylinder block in a multicylinder engine.
2. Description of the Prior Art
In a recent internal combustion engine, the increasing of the level of an output and the reducing of the weight have been demanded. Accordingly, a cylinder head and a cylinder block have been formed to smaller dimensions since the distance between adjacent cylinders, i.e. the distance between adjacent cylinder bores have increasingly been reduced. The distance between adjacent cylinder bores which is measured on the cylinder block is recently around 6 mm. Moreover, in order to further reduce the weight of an engine, the cylinder head and cylinder block tend to be produced out of an aluminum material having a low specific gravity. The low rigidity of an aluminum material and the small distance between adjacent cylinder bores have caused the rigidity of a cylinder head and a cylinder block of a recent internal combustion engine to tend to lower.
In view of such a tendency, a metallic gasket shown in FIG. 10 has been proposed. This metallic gasket has a base member 21 consisting of an elastic metallic material and provided with cylinder bore-aligned holes, i.e. combustion chamber-aligned holes 23 and beads 24 surrounding the combustion chamber-aligned holes 23, and is adapted to seal the clearance between the cylinder head and cylinder block. In the metallic gasket 20, an auxiliary member 22 is laminated on the side of the base member 21 which is on the opposite side the surface thereof from which a bead 24 projects, and the edges of the auxiliary member 22 which are on the sides of the combustion chamber-aligned holes 23 are folded back onto the surface of the base member 21 so as to wrap the edge portion of the base member 21, whereby compensating portions 25 of a predetermined thickness are provided in positions on the side of the combustion chamber-aligned holes 23 and away from the beads 24. The metallic gasket 20 formed as described above prevents a combustion gas pressure and engine heat from causing the clearance between the opposed surfaces of the cylinder block and cylinder head to be widened and narrowed, and decreases the alternating load working on the bead 24, whereby the stress on the bead 24 is reduced. Since the surface pressure is dispersed to the compensating portions, the fatigue of the metallic gasket is prevented. The compensating portions constituting primary seals function as stoppers against the deformation of the bead 24 constituting a secondary seal. Since the portions around the combustion chamber-aligned holes 23 are sealed with both the compensating portions 25 and bead 24, the leakage of a combustion gas is prevented more reliably. The metallic gaskets having the above-described structure include metallic gaskets disclosed in, for example, the specifications and drawings of Japanese Patent Laid-open No. 155376/1987 and U.S. Pat. No. 3,817,540.
In the metallic gasket 20, a compensating portion 25 constituting a grommet consists of a total of three layers including the base member 21 and a folded auxiliary member 22, i.e. a two-layer auxiliary member 22. On the other hand, a bead consists of two layers having a closed hollow space therebetween, and the portion which is between the compensation portion 25 and bead 24 consists of two layers composed of the base member 21 and auxiliary member 22 contacting each other. Accordingly, the portion of an auxiliary member 22 which is positioned on the upper side of a folded portion thereof, and a folded portion, i.e. a compensating portion 25 are exposed directly to the variation of load and vibration occurring between the cylinder head and cylinder block. Therefore, when an auxiliary member 22 formed to a small thickness is used for a long period of time, a folded portion becomes ready to be cracked. Conversely, when an auxiliary member 22 formed to a large thickness is used, a tightening load occurring when the cylinder head is tightened with respect to the cylinder block is apt to be comparatively concentrated on a three-layer compensating portion 25, so that uniform surface pressure distribution cannot be secured around the cylinder bore-aligned holes. Consequently, an excellent sealed condition cannot be maintained for a long period of time.
The conventional metallic gaskets also include a metallic gasket shown in FIG. 11. This metallic gasket 30 includes in the upper part thereof a metallic upper member 31 provided with beads 34 surrounding cylinder bore-aligned holes 33, and in the lower part thereof a lower member 32 laminated on the lower surface of the upper member 31 and bent at its edge portions on the side of the cylinder bore-aligned holes 33 toward the upper member 31 to form the cylinder bore-aligned holes 33 with shims 36 of a high rigidity provided in the spaces surrounded by the inner edge portions of the upper member 31 and the bent portions of the lower member 32. When such a metallic gasket 30 is pressed by the cylinder head and deformed to a level not lower than a predetermined level, the cylinder head contacts the shims 36. Since the shims 36 have a high rigidity, the metallic gasket 30 is not deformed any more, and it is expected that the beads 34 does not fatigue. This type metallic gaskets include a metallic gasket disclosed in, for example, Japanese Utility Model Laid-open No. 134966/1987.
In the metallic gasket 30, the shims 36 are provided immovably in the spaces surrounded by the inner edge portion of the upper member 31 and the bent portions 35 consisting of the edge portions of the lower member 32 which are on the side of the cylinder bore-aligned holes 33, and turned up toward the upper member 31. Therefore, it is necessary to prepare the shims 36 additionally, and the number of parts for manufacturing the metallic gasket 30 and the cost of manufacturing the same increase. The main portion of the lower member 32 and the portions thereof which are wrapped around the shims 36 contact the cylinder head and cylinder block as in the previously-described prior art metallic gasket. Accordingly, when the lower member 32 is formed to a small thickness, cracks are liable to occur, and, conversely, when the lower member 32 is formed to a large thickness, uniform surface pressure distribution cannot be secured around the cylinder bore-aligned holes 33.
A metallic gasket shown in FIG. 12 has also been proposed. A metallic gasket 40 consists of a base member 41 composed of an elastic metal plate, and provided thereon with a bead 44 surrounding a cylinder bore-aligned hole 43, to seal the same hole 43. An auxiliary member 42 the thickness of which is larger than that of the base member 41 is laminated on the side of the base member 41 which is opposite to the side thereof on which the bead 44 is formed, and the edge portion of the auxiliary member 42 which is on the side of the cylinder bore-aligned hole 43 is folded back on itself to form a compensating portion 45 of a predetermined thickness around the cylinder bore-aligned hole 43 with the inner end of the base member 41 and the end of the compensating portion 45 spaced from each other by a predetermined distance. Accordingly, although the bead 44 in the base member 41 is compressively deformed when the cylinder head is tightened onto the cylinder block, the folded portion 45 is not damaged and broken since the end surface of the inner circumferential portion of the base member 41 is not in contact with the folded portion 45 of the auxiliary member 42. Consequently, it can be expected that the lifetime of the metallic gasket 40 is prolonged. This type of metallic gaskets include a metallic gasket disclosed in, for example, Japanese Patent Laid-open No. 211660/1989.
In the metallic gasket 40, the thickness of the auxiliary member 42 laminated on the base member 41 provided with the bead 44 surrounding the cylinder bore-aligned hole 43 is larger than that of the base member 41. Therefore, when the cylinder head is tightened with respect to the cylinder block, a tightening load is apt to be comparatively concentrated on the compensating portion 45, so that uniform surface pressure distribution cannot be secured around the cylinder bore-aligned hole 43. The reasons reside in that, if the thickness of the bead-carrying member and that of the auxiliary member are set equal, a tightening load is not necessarily be concentrated on the folded compensating portion. It is in any case necessary in this metallic gasket 40 that the thickness of the auxiliary member 42 be set larger than that of the base member 41. Since the thickness of the auxiliary member 42 is large, the processability thereof is low, and cracks are liable to occur in the folded portion, the manufacturing cost becoming high. Moreover, a total thickness of the metallic gasket becomes large. Since the thickness of the auxiliary member 42 is large, it is difficult that the surface pressure distribution on the side of the auxiliary member 42 (cylinder block surface, for example, when the auxiliary plate 42 is disposed on the side of the cylinder block) be set proper, and it is impossible to generate the concentration of surface pressure in a predetermined position. Consequently, the surface pressure disperses more than necessary, and excellent sealed condition cannot after all be maintained.